1. Field of the Invention
The invention relates to an image forming apparatus provided with image formation controlling means for controlling a heat fixing operation for an unfixed toner image.
2. Description of Related Art
Generally, in various image forming apparatuses such as an electrophotographic copying machine and a printer, design is made such that a toner image obtained by developing is transferred onto and borne on a recording sheet, and the unfixed toner image is heated and fused by a fixing device and is fixed thereby. This fixing device, as shown, for example, in FIG. 4 of the accompanying drawings, has a fixing roller 1 as a heater disposed in a sheet transport path for a recording sheet, and a pressure roller 2 brought into pressure contact with the fixing roller 1, and when a recording sheet having an unfixed toner image thereon is passed to a nip portion which is the pressure contact portion between these two rollers 1 and 2, heating means 3 comprising a halogen heater or the like disposed in the fixing roller 1 is electrically energized, whereby the whole of the fixing roller 1 is raised in temperature, and the unfixed toner image on the recording sheet is heated and fused by the heat from the fixing roller 1, whereafter the toner image is fixed on the recording sheet.
Also, in order to improve the durability and productivity of pressure means comprising the above-described pressure roller 2, there has also been proposed a fixing device using as the pressure means a pressure belt 4 comprising a thin film-like member as shown in FIG. 5 of the accompanying drawings. This pressure belt 4 comprising a thin film-like member is brought into pressure contact with the outer peripheral surface of the fixing roller 1, and is raised in temperature by the heating means 3 disposed in the pressure contact portion being electrically energized, whereby a heat fixing operation for the unfixed toner image is likewise performed.
However, as recording sheets used in the image forming apparatus, use is made of recording sheets of various sizes from a small width size like a postcard size to a large width size such as A4-size or A3-size, and in an image forming apparatus designed to be capable of using recording sheets of those various width sizes, the above-described fixing device suffers from the following problems.
When for example, a plurality of recording sheets of a small width size like a postcard size are continuously passed to the fixing roller 1 constituting the heater of the above-described fixing device, the temperature distribution of the nip portion of the fixing roller 1 after the continuous passing of the recording sheets becomes such as shown, for example, in FIG. 6 of the accompanying drawings. That is, in the widthwisely central area in which the passing of the recording sheets of a small width size is effected, heat is taken by the passed recording sheet size, whereby the surface temperature of the fixing roller 1 in the aforementioned central area is considerably decreased. In contrast, the surface temperature of the fixing roller 1 in the widthwisely opposite end areas remains maintained at a high temperature because the passing of recording sheets is not effected there.
When with the temperature distribution of the fixing roller 1 remaining so non-uniform in the widthwise direction, a recording sheet of a large width size such as a recording sheet of A3-size or a non-fixed size is immediately passed to the fixing roller 1, the widthwisely opposite end areas which has been in a high temperature state become further overheated, and the unfixed toner image on such recording sheet is excessively fused and a so-called offset phenomenon occurs and may sometimes result in a bad image. To avoid such an offset phenomenon, when the width size of the recording sheet has been changed to a large one, an end offset avoiding mode for starting the image forming operation can be set after the surface temperature of the fixing roller 1 is decreased to a reference temperature or lower, but depending on users, there is a case where it is desired to give priority to the speed of image formation rather than to the quality of image, and it sometimes spoil operability to set the end offset avoiding mode.
So, the present invention has as its object to provide an image forming apparatus designed such that an offset phenomenon, which may occur when the width size of a recording sheet is changed, can be prevented well without spoiling a user""s operability.
In order to achieve the above object, in an image forming apparatus provided with an unfixed image forming portion for causing an unfixed toner image corresponding to an image to be borne on a recording sheet, a fixing device provided with a heater for performing a heat fixing operation to the unfixed toner image to thereby fix the toner image on the recording sheet, and image formation controlling means for controlling, on the basis of the width size of the recording sheet on which the previous image formation has been effected, and the detected temperature in the longitudinal end portion of the heater, the heat fixing operation in the next image forming operation, the image formation controlling means is provided with the end offset avoiding mode function of inhibiting the image forming operation until the detected temperature of the end portion of the heater becomes below a preset reference temperature when the width size of a recording sheet used in the next image forming operation is broader than the width size of the recording sheet on which the previous image formation has been effected and the detected temperature of the end portion of the heater is higher than the reference temperature.
Also, the image formation controlling means is provided with the function of making a performance priority mode for executing the image forming operation without inhibiting the image forming operation during the end offset avoiding mode and the end offset avoiding mode selectively changeable over.
According to such an image forming apparatus, when the width size of a recording sheet has been changed, the end offset avoiding mode is suitably executed by the image formation controlling means, whereby the so-called offset phenomenon is prevented well.
Also, depending on a user""s liking, by the performance priority mode being set, it becomes possible to give priority to the image forming operation.
Also, the image formation controlling means is provided with the function of changing over and setting one of the end offset avoiding mode and the performance priority mode in default, and also the function of effecting image formation in other mode than the mode set in default, and thereafter returning it to the mode set in default and therefore, it becomes possible to secure good usability conforming to the user""s liking.
Further, the image formation controlling means has the function of executing the end offset avoiding mode when a recording sheet used previously or used next time is a sheet of a non-fixed size and the detected temperature of the end portion of the heater is higher than the reference temperature. According to such an image forming apparatus, the so-called offset phenomenon when a sheet of a non-fixed size is used is prevented well and depending on the user""s liking, image formation giving priority to the speed of image formation becomes possible.